Field of the Invention
The invention relates to methods for magnetic resonance imaging and to corresponding magnetic resonance systems. The invention relates in particular to magnetic resonance imaging techniques in which different undersampling schemes are used for different magnetic resonance contrasts.
Description of the Prior Art
Magnetic resonance (MR) imaging techniques are known in which a result image is obtained which is indicative of a magnetic parameter in an examination region of interest. For example, the magnetic parameter can correspond to a fraction of a fat spin species or a water spin species in the examination region; in such a case reference is often made to Dixon-type techniques, which allow a water-fat separation. Generally, such techniques which permit the separation of different spin species are also referred to as chemical-shift imaging. It is also possible for the magnetic parameter to correspond e.g. to a T1 relaxation time; in such a case this is often referred to as parametric imaging.
Said aforementioned techniques are frequently based on the acquisition of a number of MR contrasts. The MR contrasts are characterized by the corresponding MR data being acquired at different time points in relation to a magnetic preparation of the magnetization. By adapting a signal model to corresponding MR data of the different MR contrasts it is then possible to determine the respective magnetic parameter.
Parallel imaging techniques (also: parallel acquisition techniques (PAT)) are known for the purpose of accelerating a period of time that is required for performing a corresponding measurement sequence (measurement duration). Typically, PAT techniques are based on an undersampling of k-space; this means that no MR data is acquired for certain k-space points; the missing information is reconstructed subsequently. The so-called acceleration factor quantifies the proportion of those k-space points along a k-space trajectory for which no MR data is acquired. Larger (smaller) acceleration factors typically correspond to a longer (shorter) measurement duration. Typically, a prior known or calibrated sensitivity profile of coils of the RF system is resorted to in order to reconstruct the missing information; this enables aliasing artifacts resulting due to the undersampling to be eliminated or reduced. Typical PAT techniques are e.g.: Generalized Auto-Calibrating Partial Parallel Acquisition (GRAPPA), see e.g. M. A. Griswold et al., in Magn. Reson. Med. 47 (2002) 1202-1210; and Sensitivity Encoding (SENSE), see e.g. K. P. Pruessmann in Magn. Reson. Med. 42 (1999) 952-962; and Simultaneous Acquisition of Spatial Harmonics (SMASH), see e.g. D. K. Sodickson and W. J. Manning in Magn. Reson. Med. 38 (1997) 591-603; and Controlled Aliasing in Volumetric Parallel Imaging (CAIPIRINHA), see e.g. F. A. Breuer et al., Magn. Reson. Med. 55 (2006) 549-556.
It is also known to apply PAT techniques in connection with the Dixon technique or parametric MR imaging. The post-processing in order to determine the result image then happens typically sequentially and separately for different picture elements (pixels) of the different contrasts; in the process, missing MR data can be reconstructed (reconstructed data) first and then the magnetic parameter can be determined by adapting the signal model to the MR data of the different contrasts.
However, such techniques have various limitations and disadvantages. The choice of larger acceleration factors of the PAT technique is often limited by the performance capability of the coil configuration of the respective MR system: a so-called g-factor can increase if a larger acceleration factor is chosen, cf. Eq. 5 from M. Blaimer et al. “SMASH, SENSE, PILS, GRAPPA: How to Choose the Optimal Method” in Top. Magn. Reson. Imaging 15 (2004) 223. The g-factor quantifies a reduction in the signal-to-noise ratio of the reconstructed data as a result of the PAT reconstruction. For this reason it is possible—e.g. given a certain reduction in the measurement duration—to obtain only a comparatively poor signal-to-noise ratio of the MR contrasts; as a result an accuracy with which the MR parameter can be determined may be limited.